The periodic table brings the chemical elements together in order of atomic number: the number of protons in an atom’s nucleus. Dmitri Mendeleev’s table of 1869 helped reveal repeating chemical patterns and left room for elements that had not yet been discovered.
Columns are called groups and rows are called periods. Elements in the same group often share chemical behaviour, while their properties change gradually across a period. The lanthanoids and actinoids are usually placed below the main table to keep its familiar shape compact.
Atomic number and atomic weight describe different things. Atomic weight reflects isotopic composition, so it is not simply a count of protons and neutrons for every atom. For elements without a standard atomic weight, a bracketed mass number identifies a selected isotope instead.
Element names preserve traces of languages, places, minerals, celestial bodies and scientists. Polonium recalls Poland, helium refers to the Sun, and mendelevium honours Mendeleev. A name may have been proposed or approved long after the element was first identified.
An element can be abundant in one setting and scarce in another. Oxygen is plentiful in the Earth’s crust, while some radioactive elements occur only in tiny transient amounts and many heavier elements are made in laboratories. Natural abundance alone does not determine price, usefulness or how easy a material is to obtain.
Sources & reference notes
Atomic weights: IUPAC 2022 abridged table; brackets show a selected isotope’s mass number. Other reference properties: PubChem. Temperatures and densities depend on pressure, phase and allotrope; missing values mean unavailable, not zero.